U.S. Pat. No. 3,642,219 discloses a known yarn feeder which has been sold by the applicant for more than 20 years as yarn feeder SFS-SFT. The backturn-detent mechanism schematically shown in U.S. Pat. No. 3,642,219 is provided between a top wall of an electro motor integrated into the storing drum and a bearing arrangement of an advance element secured to a shaft which is fixed to the housing. The detent-member co-operates with the interior circumference of the storing drum on a relatively big diameter. In this yarn feeder SFS/SFT as used in practice the detent-member is a disc made from plastic. The disc is displaceable between the running position and the braking position within a fork like counter-member secured to the shaft. The disc is displaceable by means of a drag force generated by friction contact between one front surface of the disc and the top wall of the motor. The counter-member is provided with a concave sliding track for the disc ascending in backturn direction. In the running position the disc is essentially released from its engagement with the braking surface provided at the interior periphery of the storing drum. Upon occurrence of a backturn of the storing drum the disc becomes displaced by means of the drag force alongside the slide track into the brake position, in which the disc is pressed against the braking surface. A backturn motion of the storing drum might have several reasons. A spring-loaded yarn detector at the input region of the yarn feeder is able to pull back the incoming yarn upon a stop of the storing drums and might turn back the storing drum by means of the tensed yarn. Furthermore, the yarns processed frequently are elastic. Upon a stoppage of the storing drum the tension generated beforehand in the incoming yarn tends to turn back the storing drum via the yarn. Finally, due to inappropriate circumstances and because of an intentionally controlled braking of the motor a counter-torque can be created which tends to turn back the storing drum. The known backturn-detent mechanism is complicated to manufacture, underlies significant wear and may cause vibrations when accelerating the motor or with high speeds, because the disc is active at an extremely big diameter and with a very long lever arm. The known backturn-detent mechanism requires an almost exactly vertical arrangement of the yarn feeder which cannot be achieved in some practical cases. In addition complicated structural measures are necessary to avoid burning of the disc at its front surface contacting the top wall of the motor, said burning resulting from the big distance of the contact area from the rotational axis and the thus significant friction power which has to be dissipated under high rotational speed.
It is an object of the invention to provide a yarn feeder as disclosed with a structurally simple, easy mountable backturn-detent mechanism reliably operating with reduced wear and endurance and which does not cause vibrations.